Organofluorine compounds and fluorinating agents, 10. Unusual regio‐ and stereoselective 3,4‐acetalizations of selected pyranosides by hexafluoroacetone as a result of a new reaction pathway

Abstract

AbstractRegio‐ and stereoselective direct acetalizations of chiral vicinal polyols with hexafluoroacetone (HFA) are described. Thus, the reaction of methyl α‐D‐mannopyranoside (1) and methyl a‐L‐rhamnopyranoside (5) with HFA affords cyclic 3, 4‐Acetals accompanied by simultaneous inversion of configuration at C‐3 when N, N′‐dicyclohexylcarbodiimide (DCC) is added as a coreagent. Methyl 2‐O‐(cyclohexylcarbamoyl)‐3, 4‐O‐(hexafluoroisopropylidene)‐a‐D‐altropyranoside (2) and methyl 6‐deoxy‐3, 4‐O‐(hexafluoroisopropylidene)‐a‐L‐altropyranoside (6), respectively, have been found to be the major products. The carbamoyl groups found in 2 and moreover in the byproduct 7 may be cleaved by methanolysis giving the Acetals 3 and 6, respectively. All of the products, 2, 3, 6, and 7 adopt a boat conformation as confirmed by NMR data and, additionally, ensured by X‐ray analysis in the case of 7. The incorrect structural formula for compound 6, reported as “methyl 2, 3‐O‐(hexafluoroisopropylidene)‐ct‐L‐rhamnopyranoside” in ref. [2], has to be corrected. The direct Acetalization described in this paper probably includes an intramolecular nucleophilic attack of a hemiacetal oxygen (4‐position) on C‐3 with substitution of an isourea function. magnified image